You've spotted your problem.
The vaporiser needs the heat from the coolant to work.
There is a temperature sensor on the vaporiser, when warm enough, the LPG system will run, when it's not, the system will change back to petrol.
a The coolant plumbing to the vaporiser has been altered and is no longer flowing through it correctly.
b There is a blockage, probably caused by some leak stop stuff put in the coolant or an airlock.
The vaporiser may be piped in series with the heater matrix as well. However it is piped, ditch the coolant pipe connections and flush through the vaporiser etc. with a hose pipe.
Make sure you have the correct coolant mixture when re-assembling.
That vaporiser should be hot.
To help identify your Vauxhall LPG model, go HERE